1. Field of the Invention
The invention relates to a steering control apparatus intended to operate a steering system that assists steering of a steering wheel in accordance with the steering.
2. Description of the Related Art
Japanese Patent No. 5440845 describes a control apparatus that manipulates a control angle that is input to a current feedback controller configured to allow a synchronous motor to generate an assist torque, in order to perform feedback control for causing a steering torque detected by a torque sensor to be adjusted to a target torque. The control apparatus performs torque feedback control as sensor-less control of the synchronous motor. Coordinate axes of a coordinate system that rotates based on a control angle θc are referred to as a γ axis and a δ axis. A command current value on the γ axis is larger than zero, whereas a command current value on the δ axis is zero. Thus, a q axis current flows according to the amount of shift between the γ axis and the d axis to allow a torque of the synchronous motor to be generated. The amount of shift between the γ axis and the d axis can be manipulated based on the control angle. Consequently, the control angle is manipulated based on the torque feedback control to enable manipulation of the amount of shift between the γ axis and the d axis and to allow control of the torque of the synchronous motor.
The apparatus further calculates the estimated amount of change that is the amount of rotation of the synchronous motor per predetermined time, based on an induced voltage for the synchronous motor. Based on the estimated amount of change, the apparatus then executes a guard process on the amount of update of the control angle that is the amount of operation for the torque feedback control. Japanese Patent No. 5440845 also discloses that, in this case, the estimated amount of change is corrected under a predetermined condition (paragraph [0057]).
The inventor has contrived execution of an additional guard process as a correction process for the estimated amount of change. On the other hand, in the above-described apparatus, the estimated amount of change is only utilized for the guard process for the amount of operation for the torque feedback control. Consequently, if the torque feedback control is functioning, even when the estimated amount of change fails to correspond to an exact rotation speed, the control is prevented from being disabled unless the estimated amount of change deviates excessively from the real rotation speed. However, if the torque feedback control fails to function adequately due to a small command value for a current flowing through the synchronous motor, the control angle is desirably updated in accordance with the estimated amount of control. However, in this case, when the guard process is continuously executed on the estimated amount of change, controllability of the torque of the synchronous motor may be degraded and the synchronous motor may unintentionally be subjected to regenerative control.